Fennoscandia revisited: a spatially improved tree‑ring reconstruction of summer temperatures for the last 900 years (original) (raw)
Related papers
International Journal of Climatology, 2014
Tree rings and documentary evidence are the most important palaeoclimatic archives with annual resolution that continuously span several centuries. Despite this benefit, local to regional-scale temperature reconstructions and their spatial signatures tend to be irregularly distributed, and the appropriate extent of low-frequency variability captured in these proxy records remains uncertain. Here, the first summer temperature reconstruction from the Czech Sudetes Mountains that extends to 1700 AD was introduced. An ensemble reconstruction approach using 251 new high-elevation spruce ring width samples suggests particularly cold June-July temperatures at the beginning of the 18th century, in the 1740s and around 1820. Markedly warm conditions occurred in the 1790s and during the most recent decades. The reconstructed decadal summer temperature amplitude from 'Little Ice Age Cooling' to 'Recent Anthropogenic Warming' ranges from −3.5 ∘ C between 1700 and 1710 to 1.3 ∘ C in 1999-2009, with respect to the 1961-1990 mean climatology. Comparison of our new reconstruction with existing tree-ring chronologies from the Alps reveals a significant level of coherency that is much higher than the agreement with geographically closer documentary evidence from Central Europe. Our study confirms the importance of independent regional climate reconstructions, which capture the full range of past variability and also fill spatial gaps in large-scale networks.
Last-millennium summer-temperature variations in western Europe based on proxy data
The Holocene, 2005
We present a method of multiproxy reconstruction of the climate of Europe during the last millennium. The proxies used comprise long tree-ring width series, grape harvest dates, Greenland ice oxygen isotope series and temperature indices based on historical documents. The proxies are calibrated using gridded April to September mean temperatures for western Europe, i.e., between 108W and 208E and between 358N and 558N. They are calibrated also using the long instrumental summer temperature series of the Marseilles observatory of Longchamp, which begins in the mid-eighteenth century. The method is a combination of an analogue technique, which is able to deal with missing data, an artificial neural network technique for an optimal non-linear calibration and a bootstrap technique for calculating error bars on the reconstruction. About 70% of the temperature variance is reconstructed. The amplitude of the past temperature variations is particularly well reconstructed, which is important when considering whether the recent temperature trend is or is not within the natural variability. It appears that the temperature of the last decade of the twentieth century was reached only 14 times during the last millennium. The reconstruction is discussed with respect to other multiproxy and borehole temperature reconstructions. We conclude that a reconstruction such ours, with a specific regional focus (as opposed to the larger Northern Hemisphere) is more reliable and is in better agreement with borehole results, even allowing for the fact that only a part of the long-term variance is reconstructed. 'Little Ice Age' (c. AD 1560 Á/1930) summers were 0.29/0.58C cooler than the 1961 Á/1990 period. Borehole temperatures indicate a cooling of 0.48C which falls in the 95% confidence interval of our reconstructions.
Journal of Quaternary Science, 2014
Tree-ring chronologies of maximum latewood density are most suitable to reconstruct annually resolved summer temperature variations of the late Holocene. The two longest such chronologies have been developed in northern Europe stretching back to the 2nd century BC, and the 5th century AD. We show where similarities between the two chronologies exist, and combine portions of both into a new summer temperature reconstruction for the Common Era. To minimize the transfer of potential biases, we assess the contribution of the candidate reconstructions' measurements, and exclude data (i) from exceptionally young and old trees, and (ii) produced by different laboratory technologies. Our new composite reconstruction reveals warmer conditions during Roman, Medieval and recent times, separated by prolonged cooling during the Migration period and Little Ice Age. Twentieth century warmth, as indicated in one of the existing density records, is reduced in the new reconstruction, also affecting the overall, millennial-scale, cooling trend over the late Holocene (À0.30˚C per 1000 years). Due to the reduced biological memory, typical for tree-ring density measurements, the new reconstruction is most suitable for evaluating the rate and speed of abrupt summer cooling following large volcanic eruptions.
An ensemble of European summer and winter temperature reconstructions back to 1500
Geophysical Research Letters, 2008
An ensemble of statistical methods is applied to reconstruct European temperature variability back to 1500. We apply principal component (PC) regression, regularized expectation maximization (RegEM) and composite-plus-scaling (CPS) to multi-proxy data. The reconstructions for summer and winter European temperature averages, and spatial fields related to warmest and coldest decades are analyzed and discussed. PC regression and RegEM perform more similarly compared to CPS, and more robust reconstructions are achieved for winter than for summer. We conclude that temperature reconstructions can not be improved significantly by replacing the reconstruction technique only. Discordances are very likely to be due to limited spatial and temporal availability of the proxy data. The comparison reveals that seasonal temperature variability is likely more variable than indicated earlier, still pointing out the exceptional warmth of the late 20th century. However, further evidence is needed, as the summer reconstruction results of the three techniques are not yet fully coherent.
2021
The past as an analogue for the future is one of the main motivations to use climate models for paleoclimate applications. Assessing possible model limitations in simulating past climate changes can lead to an improved understanding and representation of the response of the climate system to changes in the forcing, setting the basis for more reliable information for the future. In this study, a Regional Climate Model (RCM) is used for the investigation of the Mid-Holocene (MH, 6000 years ago) European climate, aiming to contribute to end the long-standing debate on the reconstruction of MH summer temperatures for the region, and gaining more insights on the development of appropriate methods for the production of future climate projections. Two Physically Perturbed Ensembles (PPEs) are first built by perturbing model physics and parameter values, consistently over two periods characterized by different forcing (i.e. the MH and Pre-Industrial (PI)). The goal is to uncover possible processes associated with the considered changes, that could deliver a response in MH summer temperatures closer to evidence from continental-scale proxy reconstructions. None of the investigated changes in model configuration produces remarkable differences with respect to the mean model behaviour. This indicates a limited sensitivity of the model to changes in the climate forcing, in terms of its structural uncertainty. Additional sensitivity tests are further conducted for the MH, by perturbing the model initial soil moisture conditions at the beginning of spring. A strong spatial dependency of summer near surface temperatures on the soil moisture available in spring is evinced from these experiments, with particularly remarkable differences evident over the Balkans and the areas north of the Black Sea. This emphasizes the role of soil-atmosphere interactions as one of the possible drivers of the differences in proxy-based summer temperature evident between Northern and Southern Europe. A deficiency of the considered land scheme of COSMO-CLM in properly retaining spring soil moisture, evinced from the performed tests and further confirmed by the evidence of present-day studies, suggests that the consideration of more sophisticated schemes may help bridging the gap between models and proxy-reconstructions.
Variability and extremes of northern Scandinavian summer temperatures over the past two millennia
Global and Planetary Change, 2012
Palaeoclimatic evidence revealed synchronous temperature variations among Northern Hemisphere regions over the past millennium. The range of these variations (in degrees Celsius) is, however, largely unknown. We here present a 2000-year summer temperature reconstruction from northern Scandinavia and compare this timeseries with existing proxy records to assess the range of reconstructed temperatures at a regional scale. The new reconstruction is based on 578 maximum latewood density profiles from living and sub-fossil Pinus sylvestris samples from northern Sweden and Finland. The record provides evidence for substantial warmth during Roman and Medieval times, larger in extent and longer in duration than 20th century warmth. The first century AD was the warmest 100-year period (+ 0.60°C on average relative to the 1951-1980 mean) of the Common Era, more than 1°C warmer than the coldest 14th century AD (− 0.51°C). The warmest and coldest reconstructed 30-year periods (AD 21-50 = +1.05°C, and AD 1451-80 = − 1.19°C) differ by more than 2°C, and the range between the five warmest and coldest reconstructed summers in the context of the past 2000 years is estimated to exceed 5°C. Comparison of the new timeseries with five existing tree-ring based reconstructions from northern Scandinavia revealed synchronized climate fluctuations but substantially different absolute temperatures. Level offset among the various reconstructions in extremely cold and warm years (up to 3°C) and cold and warm 30-year periods (up to 1.5°C) are in the order of the total temperature variance of each individual reconstruction over the past 1500 to 2000 years. These findings demonstrate our poor understanding of the absolute temperature variance in a region where high-resolution proxy coverage is denser than in any other area of the world.
Last nine-thousand years of temperature variability in Northern Europe
2009
The threat of future global warming has generated a major interest in quantifying past climate variability on centennial and millennial time-scales. However, palaeoclimatological records are often noisy and arguments about past variability are only possible if they are based on reproducible features in several reliably dated datasets. Here we focus on the last 9000 years, explore the results of 35 Holocene pollen-based July mean and annual mean temperature reconstructions from Northern Europe by stacking them to create summary curves, and compare them with a high-resolution, summary chironomid-based temperature record and other independent palaeoclimate records. The stacked records show that the "Holocene Thermal Maximum" in the region dates to 8000 to 4800 cal yr BP and that the "8.2 event" and the "Little Ice Age" at 500-100 cal yr BP are the clearest cold episodes during the Holocene. In addition, a more detailed analysis of the last 5000 years pinpoints centennial-scale climate variability with cold anomalies at 3800-3000 and 500-100 cal yr BP, a long, warmer period around 2000 cal yr BP, and a marked warming since the mid 19th century. The colder (warmer) anomalies are associated with increased (decreased) humidity over the Northern European mainland, consistent with the modern high correlation between cold (warm) and humid (dry) modes of summer weather in the region. A comparison with the key proxy records reflecting the main forcing factors does not support the hypothesis that solar variability is the cause of the late-Holocene centennial-scale temperature changes. We suggest that the reconstructed anomalies are typical of Northern Europe and their occurrence may be related to the oceanic and atmospheric circulation variability in the North Atlantic-North-European region.
Causes and Consequences of Past and Projected Scandinavian Summer Temperatures, 500–2100 AD
PLoS ONE, 2011
Tree rings dominate millennium-long temperature reconstructions and many records originate from Scandinavia, an area for which the relative roles of external forcing and internal variation on climatic changes are, however, not yet fully understood. Here we compile 1,179 series of maximum latewood density measurements from 25 conifer sites in northern Scandinavia, establish a suite of 36 subset chronologies, and analyse their climate signal. A new reconstruction for the 1483-2006 period correlates at 0.80 with June-August temperatures back to 1860. Summer cooling during the early 17th century and peak warming in the 1930s translate into a decadal amplitude of 2.9uC, which agrees with existing Scandinavian tree-ring proxies. Climate model simulations reveal similar amounts of mid to low frequency variability, suggesting that internal oceanatmosphere feedbacks likely influenced Scandinavian temperatures more than external forcing. Projected 21st century warming under the SRES A2 scenario would, however, exceed the reconstructed temperature envelope of the past 1,500 years.